I've had tanks like that. One or two of them were helped by a good abrasive cleaning around the rim, using a 3M green scrubbing pad. Others were bent ever-so-slightly an became utility canisters around the shop. I've also had some luck (some fit, some don't) in buying plastic tops for the SS cans, and though they don't last as long, the frustration saved is worth the extra cost of a new top every year or so.